The present invention is generally related to communication systems and, more particularly, wireless communication systems.
The purpose of a communication system is to transmit information-bearing signals from a source (transmitter) to a destination (receiver) using a channel. The transmitter processes (modulates) the message signal into a form suitable for transmission over the channel. The receiver then processes (demodulates) the received signal to reproduce an approximation of the original message signal. Modifications of the message signal achieved by modulation and numerous other suitable methods are well known in the art.
In any communication system, a key parameter which impacts system performance is the transmitter power. In a noise limited communication system, the transmitted power determines the allowable separation between the transmitter and receiver. The available transmitted power determines the signal-to-noise ratio, which must exceed some prescribed threshold at the receiver input for successful communication of information to occur.
Another key performance criterion for certain communication systems relates to the number of simultaneous users that can be accommodated. An example of one well known system application is a cellular radio telephone system. Such systems are typically comprised of a number of base sites, each having a service coverage area, and a number of mobile or hand portable cellular telephones or data terminals (hereinafter referred to as "subscribers"). The service coverage areas of base sites may be arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a subscriber receiving service from one base site may be handed off to an adjacent base site with no interruption in service. Thus, it is a key goal for a cellular, as for other wireless communication systems, to effectively utilize the available spectrum so that as many users as possible can be accommodated.
One means of accomplishing this effective utilization is through signal multiplexing, in which signals from several message sources are simultaneously transmitted over a common spectral resource. Frequency division multiplex, time division multiplex, and mixtures thereof have traditionally been used for implementing signal multiplexed cellular radio systems.
In a frequency division multiplex (FDM) or frequency division multiple access (FDMA) system, the communication spectral resource is divided into several narrow frequency bands. For at least the time needed to communicate the desired traffic, one frequency division channel is occupied by a subscriber for communication to the base site. Another frequency channel is used for traffic from the base site to the subscriber.
Time-division multiplex (TDM) systems are another type of multiple access communication system. In a TDMA (time division multiple access) system, the spectral resource is divided into repeating time frames each having a plurality of time slots or time division channels. Each time division channel is assigned to a different communication link. In this scheme, a portion of a subscriber's information occurs during an assigned slot of a frame. This is followed by one or more other time slots where information to or from other subscribers is accommodated. This process is repeated with received information being appropriately reconstructed at the receiver.
When transmitting a message signal over a communication channel, both analog and digital transmission methods can be used. At present, digital methods have become preferred due to several operational advantages over analog methods, including, inter alia: increased immunity to channel noise and interference; flexible operation of the system; common format for the transmission of different kinds of message signals; improved security of communications through the use of digital encryption; and increased capacity.
Another multiple access system involves the use of wideband communications, as opposed to narrowband approaches like FDMA and TDMA. In cellular radiotelephone systems such wideband communications have been achieved using code division multiple access (CDMA) spread spectrum techniques. Such spread spectrum systems utilize a modulation technique for spreading the information being communicated over a wide frequency band. This frequency band is typically much wider than the minimum bandwidth required to transmit the information being sent.
In a direct sequence CDMA system, communication between two communication units is accomplished by spreading each transmitted signal over a wide frequency band with a unique user spreading code. As a result, a multiplicity of transmitted signals share the same frequency. The ability of such a system to work is based on the fact that each signal is specially time and/or frequency coded to facilitate its separation and reconstruction at the receiver. Particular transmitted signals are retrieved from the communication channel by despreading a signal from the sum of signals in the communication channel with a known user spreading code related to the particular spreading accomplished at the transmitter.
In the digital direct sequence system, radio carrier modulation is performed after spreading the user's information with a digital code sequence whose bit rate is much higher than the information rate. A pseudo-random number (PN) is used as a code to "spread" the spectrum. The receiver, by utilizing the same known PN, can properly decode the received signal even when corrupted with other user's spread signals and reproduce the original information. The number of simultaneous users that can be accommodated in such a system is dependent on the amount of spectrum "spreading" that is implemented.
Another type of spread spectrum communication is "frequency hopping". In frequency hopping, the frequency of the carrier is shifted using a pattern dictated by a code sequence. The transmitter jumps from one frequency to another within some predetermined set. At the receiver, the hopping sequence for the desired user is known and allows tracking of the user's hopping transmissions. Periodically, more than one user's signal will fall on the same frequency thereby causing interference. Information coding techniques (error correction coding) are used to enable reconstruction of the original information even when a fraction of the transmitted bursts are lost. There are also time hopping and time-frequency hopping schemes whose times of transmission are regulated by the code sequence.
Still another type of spread spectrum communication is pulse-FM or "chirp" modulation, in which a carrier is swept over a wide band during a given pulse interval.
Any of the multiple access systems can be utilized in cellular radio communication systems. In such cellular systems, several factors limit performance. Typically, in propagating through the channel, a transmitted signal is distorted because of nonlinearities and imperfections in the frequency response of the channel. Other sources of degradation are noise (thermal and man made) and adjacent and co-channel interference.
Besides the typical sources of degradation mentioned above, the majority of the noise associated with a received signal in a spread spectrum CDMA system comes from the other user's signals. In systems where only spread spectrum signals are transmitted in a given frequency band (typically 1.25 MHz wide in CDMA systems), this noise comes from other user's signals which are being transmitted in the same frequency band, albeit with unique user spreading codes. In mixed wideband/narrowband systems, a spread spectrum user will also view any narrowband FDMA or TDMA signals (e.g., 30 KHz wide AMPS signals) falling within the frequency band of the spread spectrum channel as noise. Similarly, a subscriber using narrowband communications will see the spread spectrum signal as wideband noise. When either signal is transmitted without a sufficient carrier-to-interference (C/I) level with respect to the other, the first signal may be lost due to the interference from the other.
However, such mixed wideband and narrowband communications systems are likely to become more common in the future. This result is being driven by the scarcity of spectrum available for all wireless communications. With significant portions of this spectrum currently allocated to narrowband services, but with a growing demand for the type of performance promised in wideband digital systems like CDMA, a system permitting simultaneous wideband and narrowband communications within the same frequency spectrum offers a needed solution to the limited spectrum.
Consequently, there remains a need for better solutions for using wideband and narrowband communications while minimizing noise and degradations due to interference between the two types of communication.